Printing apparatus and image correction method

ABSTRACT

A printing apparatus includes a printer with printing elements that print dots and a controller that acquires input image data. The controller acquires a pre-correction printing density that is based on test pattern data, which includes a uniform array of pixels, and printing characteristics of printing positions of the dots printed by the plurality of printing elements. The controller calculates a target density by averaging the pre-correction printing density and offsets the target density so that the target density is equal to or greater than the pre-correction printing density. The controller calculates a correction gain of the plurality of printing elements on the basis of the ratio of the target density to the pre-correction printing density and controls the printer on the basis of the correction gain and the input image data.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to and the benefit of JapanesePatent Application No. 2016-127436 filed Jun. 28, 2016, the entirecontents of which are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to a printing apparatus and an imagecorrection method.

BACKGROUND

A known printing apparatus includes printing elements, and on the basisof a printed test pattern, corrects the size of the dot printed by eachprinting element so that the printing density becomes uniform. Forexample, see patent literature (PTL) 1.

CITATION LIST Patent Literature

PTL 1: JPH418363A

SUMMARY

A printing apparatus according to an embodiment of the presentdisclosure includes a printer with a plurality of printing elements thatprint dots. The printing apparatus includes a controller that acquiresinput image data. The controller acquires a pre-correction printingdensity that is based on test pattern data, which includes a uniformarray of pixels, and printing characteristics of printing positions ofthe dots printed by the plurality of printing elements. The controllercalculates a target density by averaging the pre-correction printingdensity. The controller offsets the target density so that the targetdensity is equal to or greater than the pre-correction printing density.The controller calculates a correction gain of the plurality of printingelements on the basis of the ratio of the target density to thepre-correction printing density. The controller controls the printer onthe basis of the correction gain and the input image data.

An image correction method according to an embodiment of the presentdisclosure is an image correction method for a printing apparatus. Theprinting apparatus includes a printer with a plurality of printingelements that print dots. The printing apparatus includes a controllerthat controls the printer. The image correction method includesacquiring, using the controller, a pre-correction printing density thatis based on test pattern data, which includes a uniform array of pixels,and printing characteristics of printing positions of the dots printedby the plurality of printing elements. The image correction methodincludes calculating, using the controller, a target density byaveraging the pre-correction printing density. The image correctionmethod includes offsetting, using the controller, the target density sothat the target density is equal to or greater than the pre-correctionprinting density. The image correction method includes calculating,using the controller, a correction gain of the plurality of printingelements on the basis of the ratio of the target density to thepre-correction printing density. The image correction method includescontrolling, using the controller, the printer on the basis of thecorrection gain and the input image data.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a functional block diagram schematically illustrating anexample configuration of a printing apparatus according to a firstembodiment;

FIG. 2A illustrates an example configuration of a printhead;

FIG. 2B illustrates an example configuration of a printhead;

FIG. 3 illustrates an example dot pattern printed in a grid pattern on aprinting medium;

FIG. 4 is a graph illustrating the average printing density in thescanning direction of the dot pattern in FIG. 3;

FIG. 5 is a block diagram illustrating an example of conversion of imagedata;

FIG. 6 is a graph illustrating an example relationship between inputvalues and output values of a quantizer;

FIG. 7 illustrates an example diffusion matrix;

FIG. 8 is a graph illustrating a filter coefficient of a diffusionfilter corresponding to FIG. 7;

FIG. 9A illustrates an example dot pattern printed when printingelements have uniform printing characteristics;

FIG. 9B is a graph of the printing density distribution of the dotpattern in FIG. 9A;

FIG. 10A illustrates an example dot pattern printed when printingelements exhibit variation in printing characteristics;

FIG. 10B is a graph of the printing density distribution of the dotpattern in FIG. 10A;

FIG. 11 is a graph illustrating an example setting of the target densityof the printing apparatus according to the first embodiment;

FIG. 12 is a graph illustrating an example setting of correction gain;

FIG. 13 is a graph illustrating an example setting of the target densityof an apparatus according to a comparative example;

FIG. 14 is a flowchart illustrating an example image correction methodaccording to the first embodiment;

FIG. 15 is a graph illustrating an example of applying an LPF to thewaveform of the pre-correction printing density;

FIG. 16 is a graph illustrating an example of applying an LPF to awaveform indicating the spatial distribution of correction gain; and

FIG. 17 is a graph illustrating an example of the filter coefficient ofan LPF.

DETAILED DESCRIPTION

Correcting the size of printed dots so that the printing density becomesuniform may reduce the size of dots printed by printing elements with arelatively high printing density. A reduction in the size of printeddots may yield a grainier printing result. The present embodiment makesprinting less grainy.

First Embodiment

As illustrated in FIG. 1, a printing apparatus 1 according to thepresent embodiment includes a controller 10, a memory 12, and a printer14. The printing apparatus 1 prints on a printing medium 20 with theprinter 14. The printing apparatus 1 causes a reading apparatus 30 toread the printing result on the printing medium 20. The printingapparatus 1 acquires the reading result from the reading apparatus 30.

The controller 10 acquires input image data from the memory 12 or anexternal apparatus. The controller 10 may store input image dataacquired from an external apparatus in the memory 12. On the basis ofthe input image data, the controller 10 outputs control information tothe printer 14 to cause the printer 14 to print on the printing medium20. The controller 10 can, for example, be configured by a processor ormicrocomputer capable of executing application software.

The memory 12 can be configured by a semiconductor memory or the like.Various information, programs for causing the printing apparatus 1 tooperate, and the like may be stored in the memory 12. The memory 12 mayfunction as a working memory of the controller 10.

The printer 14 includes a printing medium conveyor 16 and a printhead18. On the basis of control information from the controller 10, theprinter 14 controls the printing medium conveyor 16 and the printhead 18and prints on the printing medium 20.

The printing medium conveyor 16 conveys the printing medium 20 to theinside of the printing apparatus 1 and controls the position of theprinting medium 20 in accordance with control information from thecontroller 10. The printing medium conveyor 16 can be configured to becapable of conveying the printing medium 20 in a predetermineddirection. The “predetermined direction” may include one or a pluralityof directions.

On the basis of control information from the controller 10, theprinthead 18 prints on the printing medium 20. The printhead 18 includesprinting elements 181. The printhead 18 controls the printing elements181 to print dots on the printing medium 20. For example, the dots maybe circular, but the dots may have any other shape instead. The printingelements 181 may, for example, print by ejecting ink onto the printingmedium 20. The printing elements 181 may, for example, print bythermally transferring ink onto the printing medium 20. The printingelements 181 may print on the printing medium 20 by a variety of othermethods. The printing elements 181 may print on the printing medium 20by altering the printing medium 20.

The printing elements 181 are arrayed in the longitudinal direction ofthe printhead 18. The printing elements 181 are, for example, arrayedover a range equal to the width of the printing medium 20. The printingelements 181 may be arrayed over a range longer than the width of theprinting medium 20. The printing elements 181 may be arrayed over arange shorter than the width of the printing medium 20.

The printing elements 181 may be arrayed in a single line, asillustrated in FIG. 2A. Printing elements 181 a to 181 e are arrayed ina single line in FIG. 2A. When not distinguishing between the printingelements 181 a to 181 e, the printing elements 181 a to 181 e arecollectively referred to below as printing elements 181. The number ofprinting elements 181 is not limited to five. Four or fewer, or six ormore, printing elements 181 may be included.

As illustrated in FIG. 2B, the printing elements 181 may be arrayed overa plurality of rows, with each row being shifted a predetermineddistance. The printing apparatus 1 sequentially operates the pair ofprinting elements 181 a and 181 d, the pair of printing elements 181 band 181 e, and the printing element 181 c while shifting the relativepositions of the printhead 18 and the printing medium 20 in thetransverse direction of the printhead 18. This allows five dotscorresponding to the printing elements 181 a to 181 e to be printed in arow at a narrower pitch than the array pitch of the printing elements181.

The printer 14 sequentially prints on the printing medium 20 whilescanning the printhead 18 over the printing medium 20. The printer 14may print on the printing medium 20 by fixing the position of theprinthead 18 and displacing only the printing medium 20. The printer 14may print on the printing medium 20 by fixing the position of theprinting medium 20 and displacing only the printhead 18. The printer 14may print on the printing medium 20 by displacing both the printhead 18and the printing medium 20.

The printing medium 20 can be selected appropriately in accordance withthe printing method of the printhead 18. For example, the printingmedium 20 may be paper but is not limited to paper. Another material,such as resin, or a plurality of materials may be used. The printingmedium 20 may be in roll or sheet form or may have another shape. Theprinting medium 20 may be rectangular or have another shape, such as acircle or an ellipse.

The reading apparatus 30 includes a light source that irradiates lightonto a reading target and a sensor that detects reflected light orscattered light from the reading target. The reading apparatus 30 canread the printing result from the printing medium 20. The readingapparatus 30 outputs the reading result to the printing apparatus 1. Thereading result includes information pertaining to the densitydistribution of the printing result.

The printing elements 181 have printing characteristics. The printingcharacteristics of the printing elements 181 include data pertaining tothe gradations of density representable with dots printed by theprinting elements 181. The gradations of density are, for example,determined by the size of dots. The gradations of density may bedetermined by the printing array of dots.

The printing characteristics include correction data indicating thedifference between a printing result predicted on the basis of controlinformation output by the controller 10 and the actual printing result.The correction data may include the printing positions of dots. Thecorrection data on the printing position of dots defines the degree ofshifting in the position at which dots are printed as compared to theprinting position of dots predicted from the control information. Thecorrection data may include the size of the printed dots. The correctiondata on the dot size defines the degree of difference in the size ofprinted dots as compared to the dot size predicted from the controlinformation.

The correction data can be acquired from a test pattern printed on theprinting medium 20. The printing apparatus 1 acquires test pattern dataas input image data and prints a test pattern on the printing medium 20.The printing apparatus 1 causes the reading apparatus 30 to read theprinted test pattern and acquires the result of reading the testpattern. The printing apparatus 1 analyzes the reading result of thetest pattern and acquires the correction data of each printing element181. The correction data of each printing element 181 can also beconsidered inspection data of each printing element 181 acquired fromthe printing result of the test pattern data.

The test pattern includes a printing pattern for acquiring thecorrection data of each printing element 181. For example, the testpattern may include a pattern allowing measurement of the shift inprinting position of the dot of each printing element 181. The patternallowing measurement of the shift in printing position of a dot may, forexample, include a pattern indicating coordinates. The printingapparatus 1 can use the reading result of the pattern allowingmeasurement of the shift in printing position of a dot to calculate thedifference between the position of the dot printed by each printingelement 181 and the position predicted from the control information. Theprinting position of a dot may be acquired from the density distributionof the printed dot pattern. The printing position of a dot may beacquired from the printing result of a pattern that has a scale allowingmeasurement of the printing position. A variety of other methods can beused as the method of acquiring the printing position of a dot.

The test pattern may include a pattern allowing measurement of the sizeof the dot of each printing element 181. The pattern allowingmeasurement of the size of a dot may, for example, include a patternindicating a scale. The printing apparatus 1 can use the reading resultof the pattern allowing measurement of the size of a dot to calculatethe difference between the size of the dot printed by each printingelement 181 and the size predicted from the control information. The dotsize may be acquired by measuring the density of the printed dotpattern. The dot size may be acquired by directly measuring anindividual printed dot with a microscope or the like. A variety of othermethods can be used as the method of acquiring the dot size.

The printing apparatus 1 may include either or both of the printingmedium 20 and the reading apparatus 30. In this case, the printingapparatus 1 can print the test pattern and read the printing resultinternally to acquire the correction data of the printing elements 181without using an external apparatus.

The input image data acquired by the printing apparatus 1 may, forexample, be in bitmap format or any other format. The printing apparatus1 according to the present embodiment is assumed to acquire input imagedata formed by pixels arranged in a grid pattern in bitmap format or thelike. For example, the printing apparatus 1 according to the presentembodiment prints on the basis of input image data so that dots arearrayed in a grid pattern on the printing medium 20. The dot arrayprinted by the printing apparatus 1 is not limited to being a gridpattern and may have randomness. The printing apparatus 1 forms aprinting result corresponding to the input image data by printing acollection of dots on the printing medium 20.

As illustrated in FIG. 3, the dots may be printed at the intersectionsin a grid formed by dashed dotted lines extending in the X-axisdirection and the Y-axis direction. The X-axis direction and the Y-axisdirection are also referred to as the X-direction and the Y-direction.

The dot pattern in the example in FIG. 3 may appear to the human eye asa different pattern depending on the dot size and the array spacing. Forexample, each dot may be distinguishable when the array spacing is widerelative to the dot size. When the array spacing is narrow relative tothe dot size, the dot pattern may appear to be a uniform-density solidpattern in which individual dots are not distinguishable. The spatialfrequency of the dot pattern is determined by the dot size and the arrayspacing. In other words, the dot pattern may appear to the human eye asa different pattern depending on the spatial frequency.

The way the dot pattern appears to the human eye can be evaluated by thecontrast sensitivity characteristics of the human eye. The contrastsensitivity characteristics are also referred to as contrast sensitivityfunction (CSF) characteristics. The CSF characteristics represent therelationship between the spatial frequency of the dot pattern and thecontrast sensitivity of the human eye.

Fluctuation in printing density due to the dot pattern is easy todistinguish when the spatial frequency of the dot pattern is in afrequency band with increased contrast sensitivity of the human eye. Inthis case, dots tend to be easier to distinguish individually. Such anappearance is also referred to as being grainy.

Fluctuation in printing density due to the dot pattern is hard todistinguish when the spatial frequency of the dot pattern is in afrequency band with low contrast sensitivity of the human eye. In thiscase, the dot pattern tends to appear as a uniform-density solid patternin which individual dots are not distinguishable. Such an appearance isalso referred to as not being grainy or being slightly grainy. Ingeneral, fluctuation in printing density due to the dot pattern is easyto distinguish when the spatial frequency of the dot pattern is includedin a predetermined frequency band. On the other hand, fluctuation inprinting density due to the dot pattern is hard to distinguish when thespatial frequency of the dot pattern is not included in a predeterminedfrequency band, i.e. when the spatial frequency is included in afrequency band that is higher or lower than a predetermined frequencyband.

The spatial frequency of the dot pattern may be calculatedtwo-dimensionally for each of the X-direction and the Y-direction inFIG. 3, for example. The spatial frequency of the dot pattern may becalculated one-dimensionally for the dots in a certain row of the dotpattern. In the present embodiment, the spatial frequency relative tothe X-direction array of the dot pattern is calculated using the averageprinting density of dots arrayed in the Y-direction in FIG. 3.

As illustrated in FIG. 4, the average printing density in theY-direction of the dot pattern in FIG. 3 changes in accordance with theposition in the X-direction. The horizontal axis in FIG. 4 representsthe position in the X-direction. The vertical axis represents theaverage printing density in the Y-direction. The printing density has adistribution corresponding to the dot array. The average printingdensity calculated along lines in the Y-direction traversing the centerof dots corresponds to local maxima in the graph in FIG. 4. The linestraversing the center of dots in the Y-direction are indicated by dasheddotted lines in FIG. 3. On the other hand, the average printing densitycalculated along lines in the Y-direction traversing portions where nodots are printed corresponds to local minima in the graph in FIG. 4. Thelines in the Y-direction traversing portions where no dots are printedare not illustrated in FIG. 3. In FIG. 3, portions where dots are notprinted exist over a predetermined width in the X-direction. The averageprinting density of a portion where no dots are printed may be the samelocal minimum over the predetermined width. When the average printingdensity is the same local minimum over a predetermined width, the graphnear the local minimum of the average printing density may become astraight line indicating a constant value. Depending on the actualresolution at which the reading apparatus 30 reads the printing result,the graph near the local minimum of the average printing density becomesa curve, as illustrated in FIG. 4.

The spatial frequency in the X-direction of the dot pattern in FIG. 3 iscalculated by performing frequency spectrum analysis on the waveform ofthe average printing density illustrated in FIG. 4. When the calculatedspatial frequency is included in a frequency band with high contrastsensitivity for the human eye in terms of CSF characteristics, the humaneye can distinguish between dots corresponding to the local maxima anddots corresponding to the local minima of the average printing densityin FIG. 4. In other words, each dot in the dot pattern in FIG. 3 isvisibly distinguishable. When the calculated spatial frequency isincluded in a frequency band with low contrast sensitivity for the humaneye in terms of CSF characteristics, the human eye can no longerdistinguish between dots corresponding to the local maxima and dotscorresponding to the local minima of the average printing density inFIG. 4. In this case, the dot pattern appears to be printed as a solidpattern with uniform printing density, like the apparent densityindicated by the dashed line in FIG. 4.

The dot size and array spacing that determine the spatial frequency ofthe dot pattern may be changed by the printing apparatus 1 over apredetermined range. The dot size may, for example, be changed bycontrolling the amount of ink ejected from the printing elements 181.The dot spacing interval can be determined by the spacing interval ofthe printing elements 181 provided in the printhead 18. The dot spacinginterval can also be changed by controlling the relative positions ofthe printhead 18 and the printing medium 20 in the array direction ofthe printing elements 181.

The printing apparatus 1 converts the input image data to output imagedata on the basis of the printing characteristics of the printingelements 181 to improve the degree of reproduction of the printingresult relative to the input image data. When the number of gradationsrepresentable by dots printed by the printing elements 181 is smallerthan the number of gradations of the input image data, the printingapparatus 1 generates output image data by color reduction that reducesthe number of gradations. An error diffusion method or the like, forexample, may be used for color reduction. The processing to generate theoutput image data is not limited to color reduction. The printingapparatus 1 may, for example, generate output image data that increasesthe printing speed. The printing speed can also be thought of as thenumber of dots printed per unit time. The printing apparatus 1 maygenerate the output image data in accordance with image conversionsettings from the user, such as settings for brightness, saturation, orthe like, or settings for image sharpness. The controller 10 may storethe generated output image data in the memory 12.

When the image data targeted for color reduction is a monochrome image,the number of gradations in the density of black may be reduced. Whenthe image data targeted for color reduction is a color image, the numberof gradations in the density of each of the primary colors for printing,including colors such as cyan, magenta, and yellow, may be reduced.

As illustrated in FIG. 5, the block that converts image data includes amultiplier 51, a first calculator 52, a quantizer 53, a secondcalculator 54, and a filter 55. The block that converts image data isalso referred to as a conversion block. The first calculator 52 and thesecond calculator 54 may, for example, each be formed by adders. Thefunctions of each component of the conversion block can be executed bythe controller 10. The components of the conversion block can beimplemented as discrete constituent elements.

The conversion block of the controller 10 sequentially converts thepixels of the input image data one pixel at a time into pixels of outputimage data. One pixel of input image data is represented as u. One pixelof input image data is also referred to below as input pixel data. Onepixel of output image data is represented as y. One pixel of outputimage data is also referred to below as output pixel data. Thecontroller 10 is assumed to convert pixels sequentially to the rightfrom the pixel in the upper left corner of the input image data. Afterconverting up to the pixel at the right edge, the controller 10 convertsthe pixels one row lower from left to right. The controller 10 convertspixels sequentially until converting the pixel in the lower rightcorner. The controller 10 may convert pixels in a different order.

The multiplier 51 multiplies the input pixel data by a predeterminedcoefficient and outputs the result. The predetermined coefficient isrepresented as a. The predetermined coefficient is also referred to ascorrection gain and is a positive real number. The correction gain isdetermined by the below-described density distribution correction.

The first calculator 52 outputs the difference between the output of themultiplier 51 and the output of the filter 55. The output of the filter55 is data that provides feedback on the quantization error generated bycolor reduction, with an error diffusion method or the like, of otherinput pixel data processed before the input pixel data inputted to themultiplier 51. The output of the first calculator 52 becomes data thatincludes the quantization error generated in other input pixel data. Theoutput of the first calculator 52 is represented as ϕ.

The quantizer 53 receives the output (ϕ) of the first calculator 52 asan input value and outputs an output value yielded by quantizing theoutput (ϕ). In other words, the quantizer 53 adds a quantization error(n) to the output (ϕ) and outputs the result as output pixel data.

On the basis of the relationship between input values and output valuesillustrated in the graph in FIG. 6, for example, the quantizer 53converts an input value to an output value. In FIG. 6, the relationshipbetween input values and output values is indicated by bold lines. Theoutput value of the quantizer 53 for an input value of at least 0 andless than 42 is, for example, 0. The output value of the quantizer 53for an input value of at least 42 and less than 127 is, for example, 85.The output value of the quantizer 53 for an input value of at least 127and less than 212 is, for example, 170. The output value of thequantizer 53 for an input value of at least 212 and 255 or less is, forexample, 255.

According to the example relationship in FIG. 6, the output value of thequantizer 53 is 85 for an input value of 100, for example. In this case,the quantization error is −15.

The second calculator 54 outputs the difference between the output (y)of the quantizer 53 and the output (ϕ) of the first calculator 52. Theoutput of the second calculator 54 corresponds to the quantization errorgenerated when ϕ was quantized.

The filter 55 diffuses the quantization error, generated when ϕ wasquantized, to other pixels. The circuit including the filter 55 providesfeedback on the conversion result for a certain pixel to conversion ofother pixels.

The filter 55 uses a diffusion matrix, for example, to diffuse thequantization error to pixels surrounding the pixel for which thequantization error was generated. As illustrated in FIG. 7, thediffusion matrix may have a plurality of cells. The arrangement of cellsin the diffusion matrix corresponds to the arrangement of pixels in theimage data. In the diffusion matrix of FIG. 7, the cell indicated by anasterisk (*) is a diffusion source cell 60 corresponding to a pixel thatis the diffusion source of quantization error. The pixel that becomesthe diffusion source of quantization error is also referred to as adiffusion source pixel. The adjacent cell to the right of the diffusionsource cell 60 corresponds to the adjacent pixel to the right of thediffusion source pixel. The number of cells forming the diffusion matrixis not limited to a total of three rows by five columns as in FIG. 7.The cells may be in two or fewer rows, or four or more rows. The cellsmay be in four or fewer columns, or six or more columns. The arrangementof cells is not limited to a matrix. The cells may be arranged in anyway, such as stepwise or as an inverted pyramid.

The numerical values indicated in the cells of the diffusion matrix areweights used when diffusing error. In the diffusion matrix in FIG. 7, aweight is given to each cell. The cells that have already been convertedto the left of the diffusion source cell 60 are not given weights. Thefilter 55 allocates the quantization error of the diffusion source pixelto the cells in proportion to the weights given to the cells. In otherwords, the quantization error of the diffusion source pixel is allocatedto the cells in accordance with the ratio between the weight given toeach cell and the sum of the weights.

In the example in FIG. 7, the sum of the 12 weights is 47. The cell tothe right of the diffusion source cell 60 is given a weight of 7. Thecell to the right of the diffusion source cell 60 is also referred to asa first diffusion target cell 61. The filter 55 allocates 7/47 of thequantization error of the diffusion source pixel to the first diffusiontarget cell 61. The cell two cells to the right and two cells below thediffusion source cell 60 is given a weight of 1. The cell two cells tothe right and two cells below the diffusion source cell 60 is alsoreferred to as a second diffusion target cell 62. The filter 55allocates 1/47 of the quantization error to the second diffusion targetcell 62. The filter 55 allocates the quantization error similarly to theother cells. With this approach, the sum of the quantization errorallocated to the cells in the diffusion matrix is equal to thequantization error of the diffusion source pixel.

When a cell has no corresponding pixel, the quantization error allocatedto that cell need not be provided as feedback to the conversion of theinput pixel data. The case of a cell having no corresponding pixelincludes, for example, the diffusion source pixel being a pixel in thebottom row.

When a cell has no corresponding pixel, the filter 55 may be configurednot to allocate the quantization error to the cell. The quantizationerror allocated to other cells in this case may become relatively large.

The processing by the filter 55 allocates a quantization error to eachpixel from a plurality of diffusion source pixels. For each pixel, thefilter 55 accumulates and stores the quantization error allocated to thepixel from the plurality of diffusion source pixels. The filter 55 maystore the quantization error allocated to each pixel in the memory 12.The filter 55 outputs, to the first calculator 52, the quantizationerror allocated to the pixel that is input to the first calculator 52for conversion.

The filter 55 can also be implemented by applying a diffusion filter tothe image data. The diffusion matrix in FIG. 7 is associated with adiffusion filter having the filter characteristics illustrated in FIG.8. In FIG. 8, the horizontal axis represents the horizontal position ofthe pixel to be handled during conversion, and the vertical axisrepresents the filter coefficient corresponding to each position.Position 3 on the horizontal axis corresponds to the third column thatincludes the diffusion source cell 60 in the diffusion matrix. Position1 corresponds to the first column located furthest to the left in thediffusion matrix. Similarly, positions 2, 4, and 5 respectivelycorrespond to the second, fourth, and fifth columns of the diffusionmatrix.

The quantization error of the diffusion source pixel is diffused to thefirst through fifth columns in accordance with the filtercharacteristics illustrated in FIG. 8. As illustrated in FIG. 8, thequantization error of the diffusion source pixel is diffused more to thefourth column than to the third column. In other words, the diffusiontarget of the quantization error is shifted to the column on the right.The quantization error diffused to the first or fifth columns isrelatively small.

Diffusing the quantization error to surrounding pixels can be thought ofas cutting the high-frequency component of the spatial frequencyspectrum of the image data. In other words, the diffusion filter hasfrequency characteristics that allow a low-frequency component to pass.Such frequency characteristics that allow a low-frequency component topass can be considered the characteristics of a low pass filter (LPF).The image data processed by the diffusion filter is made up of frequencycomponents that include a frequency band with low contrast sensitivityin terms of CSF characteristics. This approach makes fluctuation in theprinting density less noticeable to the human eye, even forcolor-reduced image data.

The printing apparatus 1 can drive the plurality of printing elements181 of the printhead 18 in parallel. For example, the printing apparatus1 can drive five printing elements 181 arrayed in a line (see FIG. 2A)in parallel to print five dots simultaneously. The printing apparatus 1can drive the printing elements 181 while scanning the printhead 18 overthe printing medium 20 to sequentially print lines of dots. For example,when the printing elements 181 are driven three times, a 3×5 dot patternsuch as the example in FIG. 9A is printed. This allows the printingapparatus 1 to increase the printing speed.

In FIG. 9A, the direction in which five dots are aligned is assumed tobe the X-direction. The direction in which three dots are aligned isassumed to be the Y-direction. The X-direction corresponds to thelongitudinal direction of the printhead 18. In other words, theX-direction corresponds to the array direction of the printing elements181. The Y-direction corresponds to the direction in which the printhead18 is scanned over the printing medium 20. In other words, theY-direction corresponds to the scanning direction of the printhead 18.

In the present embodiment, the dots are sequentially printed from bottomto top in the Y-direction. The dots may be sequentially printed from topto bottom. The rows and columns of the dot pattern may be interchanged.The dots may be printed in any of various orders other than the onesdescribed above. The number of printing elements 181 is not limited tofive. Four or fewer, or six or more, printing elements 181 may beincluded.

In FIG. 9A, scanning lines 182 a to 182 e indicating the scanningdirection are indicated by dashed dotted lines in the Y-direction. Thescanning lines 182 a to 182 e are also referred to as scanning lines182. The dots printed along the scanning line 182 a are printed by theprinting element 181 a. The dots printed along each of the scanninglines 182 b to 182 e are respectively printed by the printing elements181 b to 181 e.

In FIG. 9A, printing lines 183 a to 183 c are indicated by dashed lines.The printing lines 183 a to 183 c are also referred to as printing lines183. The printing lines 183 are targets for matching the positionbetween the printhead 18 and the printing medium 20. The printingapparatus 1 prints dots at the intersections between the scanning lines182 and the printing lines 183.

The dots in the example in FIG. 9A have uniform size and are printedwithout deviating from the intersections between the scanning lines 182and the printing lines 183. On the basis of uniform input image data,this type of dot pattern can be printed by printing elements 181 havinguniform printing characteristics.

FIG. 9B is a graph plotting the average printing density in the scanningdirection calculated over paths along the scanning direction(Y-direction) of the printhead 18 for the dot pattern in FIG. 9A. Thehorizontal axis represents the position in the X-direction. The verticalaxis represents the printing density. Points A to E on the horizontalaxis respectively correspond to positions in the X-direction of thescanning lines 182 a to 182 e. For example, the average printing densityin the scanning direction at point A is calculated over the path of thescanning line 182 a.

The spatial frequency in the X-direction of the dot pattern in FIG. 9Ais calculated by performing frequency spectrum analysis on the waveformof FIG. 9B. The spatial frequency spectrum in the X-direction of the dotpattern in FIG. 9A is, for example, mainly made up of a frequencycomponent determined using the interval of the scanning lines 182 as theperiod and overall is included in a frequency band with lower contrastsensitivity of the human eye in terms of CSF characteristics. In thiscase, the dot pattern in FIG. 9A appears to be a uniform-density solidpattern in the X-direction, like the apparent density indicated by thedashed line in FIG. 9B. When the frequency determined using the intervalof the printing lines 183 as the period is included in a frequency bandwith lower contrast sensitivity of the human eye, the dot patternappears to be a uniform-density solid pattern in the Y-direction.

The correction data included in the printing characteristics of eachprinting element 181 may vary for numerous reasons. For example, theprinting elements 181 may produce variation in the printing positions ofdots due to error in the array positions of the printing elements 181.The printing density may also vary due to variation in the dot sizecaused by variation in the amount of ink in a drop. The causes forvariation in the correction data are not limited to the above examples.

When there is variation in the correction data of the printing elements181, there may be variation in the printing result. Even when printingis based on uniform input image data, the printing positions of theprinted dots will not be uniform, as in the example in FIG. 10A, if thecorrection data pertaining to the printing positions of dots variesbetween printing elements 181, for example. The printing positions ofdots in FIG. 10A are shifted to the left by the scanning line 182 b tobe closer to the scanning line 182 a, and shifted to the right by thescanning line 182 d to be closer to the scanning line 182 e. In otherwords, the printing result in the example in FIG. 10A appears to havestreaks along the scanning direction.

The variation in correction data pertaining to the printing positions ofdots is, for example, evaluated using the average printing density inthe scanning direction calculated over a path along the scanningdirection indicated by the scanning lines 182. FIG. 10B is a graphplotting the average printing density in the scanning directioncalculated for paths along the scanning direction (Y-direction) of theprinthead 18 for the example dot pattern in FIG. 10A. The horizontal andvertical axes are the same as in FIG. 9B.

The spatial frequency in the X-direction of the dot pattern in FIG. 10Ais calculated by performing frequency spectrum analysis on the waveformof FIG. 10B. The spatial frequency spectrum in the X-direction of thedot pattern in FIG. 10A includes a frequency component determined usingthe interval of the scanning lines 182 as the period and a frequencycomponent determined by the fluctuation resulting from variation in theprinting positions of dots printed on each scanning line 182. As in FIG.9B, the frequency component determined using the interval of thescanning lines 182 as the period is included in a frequency band withlower contrast sensitivity of the human eye in terms of CSFcharacteristics. On the other hand, the frequency component determinedby the fluctuation resulting from variation in the printing positions ofdots is assumed to be included in a frequency band in which the contrastsensitivity is higher. In this case, the dot pattern in FIG. 10A appearsto be a pattern with a density distribution in the X-direction like theapparent density indicated by the dashed line in FIG. 10B. In otherwords, the example dot pattern in FIG. 10A appears to have a relativelyhigh density at point A and point E and a relatively low density atpoint B and point D in the X-direction. The dot pattern in FIG. 10Aappears to have low-density streaks at positions along the scanningdirection corresponding to positions between point B and point C andbetween point C and point D. The dot pattern in FIG. 10A appears to havehigh-density streaks at positions along the scanning directioncorresponding to positions between point A and point B and between pointD and point E.

In FIG. 9B and FIG. 10B, the apparent density indicated by the dashedline can be calculated by applying an LPF to the average printingdensity in the scanning direction indicated by the solid line, the LPFhaving frequency characteristics that take CSF characteristics intoaccount. The application of an LPF to the average printing density inthe scanning direction can remove the noise component included in theaverage printing density in the scanning direction.

The apparent density can also be calculated by calculating a movingaverage over set predetermined sections of the average printing densityin the scanning direction. The predetermined sections are, for example,set to the intervals of the scanning lines 182. Calculating a movingaverage of the average printing density in the scanning direction canalso remove the noise component included in the average printing densityin the scanning direction.

The example of the average printing density in the scanning direction inFIG. 10B may, for example, be acquired from the printing result of inputimage data that is predicted to appear as having uniform density. Theinput image data for which the printing result is predicted to appear ashaving uniform density is, for example, test pattern data that includesa uniform array of pixels. The printing apparatus 1 causes the readingapparatus 30 to read the printing result that was printed on theprinting medium 20 on the basis of input image data including a uniformarray of pixels. The printing apparatus 1 can calculate the averageprinting density in the scanning direction from the reading resultacquired from the reading apparatus 30. The printing density thusmeasured from the actual printing result is also referred to as themeasured printing density. The printing apparatus 1 may predict theprinting density of the printing result on the printing medium 20 on thebasis of the printing characteristics of each printing element 181 andthe input image data. The printing density thus predicted is alsoreferred to as the predicted printing density.

When the printing result differs from the result predicted from theinput image data, the controller 10 of the printing apparatus 1 caninclude data for correcting the printing result in control informationgenerated for the printing elements 181. The printing result may be themeasured printing density or the predicted printing density. Themeasured printing density or the predicted printing density is alsoreferred to as the pre-correction printing density. The case of theprinting result differing from the result predicted from the input imagedata may, for example, correspond to the printing result being predictedto appear as a uniform-density solid pattern but not appearing so, as inFIG. 10B.

When the printing result differs from the predicted result due to theprinting characteristics pertaining to the printing positions of thedots, the controller 10 may generate control information for theprinting elements 181 on the basis of the correction data pertaining tothe printing positions of the dots of the printing elements 181. In thiscase, the controller 10 may set a correction gain as the predeterminedcoefficient (a) of the multiplier 51 in the conversion block of FIG. 5.The correction gain set during the conversion of a pixel printed by aprinting element 181 is also referred to as the correction gain of theprinting element 181.

In FIG. 10A, the dots along the scanning line 182 b printed by theprinting element 181 b are shifted to the left, closer to the scanningline 182 a. Consequently, in FIG. 10B, the apparent density at point Abecomes relatively higher. The apparent density at point B becomesrelatively lower. For the printing result to appear to the human eye tohave uniform density, the printing apparatus 1 may generate controlinformation to cause the printing element 181 b or printing element 181c to print a larger sized dot than the dot based on the input pixeldata. In this case, the printing apparatus 1 generates output pixel dataof the pixel corresponding to the printing element 181 b or printingelement 181 c after setting the correction gain to a >1 when the inputpixel data of the pixel corresponding to the printing element 181 b orprinting element 181 c is input into the multiplier 51. The printingapparatus 1 may generate control information to cause the printingelement 181 a to print a smaller sized dot than the dot based on theinput pixel data. In this case, the printing apparatus 1 generatesoutput pixel data of the pixel corresponding to the printing element 181a after setting the correction gain to a <1 when the input pixel data ofthe pixel corresponding to the printing element 181 a is input into themultiplier 51.

In FIG. 10A, the dots along the scanning line 182 d printed by theprinting element 181 d are shifted to the right, closer to the scanningline 182 e. Consequently, in FIG. 10B, the apparent density at point Ebecomes relatively higher. The apparent density at point D becomesrelatively lower. For the printing result to appear to the human eye tohave uniform density, the printing apparatus 1 may generate controlinformation to cause the printing element 181 c or printing element 181d to print a larger sized dot than the dot based on the input pixeldata. In this case, the printing apparatus 1 generates output pixel dataof the pixel corresponding to the printing element 181 c or printingelement 181 d after setting the correction gain to a >1 when the inputpixel data of the pixel corresponding to the printing element 181 c orprinting element 181 d is input into the multiplier 51. The printingapparatus 1 may generate control information to cause the printingelement 181 e to print a smaller sized dot than the dot based on theinput pixel data. In this case, the printing apparatus 1 generatesoutput pixel data of the pixel corresponding to the printing element 181e after setting the correction gain to a <1 when the input pixel data ofthe pixel corresponding to the printing element 181 e is input into themultiplier 51. As in the example above, the method of setting thecorrection gain may thus be based on the printing density resulting froma shift in the printing positions of dots.

In the present embodiment, the correction gain is set in accordance witha target value of the printing density to be obtained after correction.The target value of the printing density is also referred to as thetarget density. The target density may be the printing density whenthere is no difference between the printing result predicted on thebasis of the control information for the printing elements 181 and theactual printing result of the printing elements 181. The target densitymay be the average printing density in the scanning direction, or theaverage apparent density, at each position in the X-direction. Thetarget density may be set to a value corresponding to the maximum orminimum of the average printing density in the scanning direction or theapparent density. The target density may be set to a value equal to orgreater than the maximum of the apparent density. The target density maybe any value other than the above-described values.

In the present embodiment, the target density is set to a value yieldedby adding a density offset to the average density obtained by averagingthe apparent density at each position in the X-direction, as illustratedin FIG. 11, for example. Adding the density offset to the target densityis also referred to as offsetting the target density. In FIG. 11, thetarget density is indicated by a solid line, the average density by adashed dotted line, and the apparent density by a dashed line. Thedensity offset may be calculated as the maximum of the differencebetween the average density and the apparent density before averaging.The target density set in this way becomes equal to or greater than theapparent density at every position in the X-direction. The densityoffset may be set to the amplitude of the waveform yielded by thedifference between the waveform of the average density and the waveformof the apparent density before averaging.

Calculating the apparent density by applying an LPF to the averageprinting density in the scanning direction can be considered a type ofaveraging. Calculating the average printing density in the scanningdirection from the pre-correction printing density can also beconsidered a type of averaging. Calculating the average density from theapparent density can also be considered a type of averaging. In otherwords, the average density can be calculated by averaging thepre-correction printing density.

When the target density is set as illustrated in FIG. 11, the apparentdensity at point B, for example, is lower than the target density. Inthis case, the printing apparatus 1 sets the correction gain forgenerating the output pixel data for the printing element 181 bcorresponding to point B to a value greater than 1. The printingapparatus 1 also sets the correction gain to a value greater than 1 forpoints C and D, where the apparent density is lower than the targetdensity, like point B. On the other hand, the apparent density at pointA is substantially equal to the target density. In this case, theprinting apparatus 1 sets the correction gain for generating the outputpixel data for the printing element 181 a corresponding to point A to 1.The printing apparatus 1 also sets the correction gain to 1 for point E,where the apparent density is substantially equal to the target density,like point A.

The correction gain can be set as illustrated in FIG. 12. The horizontalaxis of the graph in FIG. 12 represents the position in the X-direction.The vertical axis of the graph in FIG. 12 represents the correctiongain. The line where the correction gain is 1 is indicated by the dasheddotted line. The example of setting the correction gain corresponding tothe target density in FIG. 11 is indicated by a solid line for the caseof an offset. In this case, the correction gain is set to a value of 1or greater at every position in the X-direction.

In the present embodiment, the correction gain is assumed to be set to avalue of 1 or greater for the printing elements 181 corresponding toevery position in the X-direction. This approach makes it possible forthe size of the dots printed by the printing elements 181 not to besmaller than in the case of no correction.

An apparatus according to a comparative example sets the target densityto the average density, as illustrated in FIG. 13. In this case, theapparent density at point B, for example, is lower than the targetdensity. The apparatus of the comparative example sets the correctiongain of the printing element 181 b to a value greater than 1 in thiscase. On the other hand, the apparent density at point A is higher thanthe target density. The apparatus of the comparative example sets thecorrection gain of the printing element 181 a to a value less than 1 inthis case.

The correction gain set in correspondence with the target densityillustrated in FIG. 13 is indicated in FIG. 12 by a dashed line as thecase of no offset. In this case, the correction gain is greater or lessthan 1 depending on the position in the X-direction. In the comparativeexample, the correction gain is set to a value less than 1 for at leasta portion of the printing elements 181.

When the correction gain corresponding to a printing element 181 is setto a value less than 1, the size of the dot printed by the printingelement 181 may be smaller than in the case of no correction. When thedot size becomes smaller, the space from an adjacent printed dotincreases. The dot pattern printed in this way may have a high contrastsensitivity. Consequently, the printing result tends to become grainy.

As described above, the printing apparatus 1 according to the presentembodiment can generate output image data that is corrected so that theprinting result has less of a tendency to appear grainy than with theapparatus of the comparative example.

An example of the image correction method executed by the printingapparatus 1 of the present embodiment is described with reference to theflowchart in FIG. 14.

The controller 10 of the printing apparatus 1 prints a test pattern onthe printing medium 20 (step S1). Information pertaining to the printingcharacteristics of each printing element 181 is included in the printingresult of the test pattern.

The controller 10 acquires the density distribution in the printingresult of the test pattern from the reading apparatus 30 (step S2). Thereading apparatus 30 reads the density distribution in the printingresult of the test pattern from the printing medium 20 and outputs thedensity distribution to the controller 10. The controller 10 may storethe acquired density distribution in the memory 12.

The controller 10 averages the density distribution (step S3). Thedensity distribution is, for example, represented as the averageprinting density in the scanning direction illustrated in FIG. 10B. Theaverage printing density in the scanning direction is represented as acomposite waveform of the fluctuating component resulting from variationin the size of dots printed by the scanning lines 182 and thefluctuating component having the interval of the scanning lines 182 asthe period. The controller 10 analyzes the spatial frequency spectrum inthe X-direction for the average printing density in the scanningdirection in FIG. 10B and then calculates the apparent density byfiltering based on CSF characteristics. The filtering based on CSFcharacteristics is a type of averaging. The controller 10 averages theapparent density to calculate the average density. The averaging of theapparent density may, for example, simply be a calculation of theaverage of the apparent density at each position in the X-direction. Theaveraging of the apparent density may be a calculation of a movingaverage using predetermined sections. The predetermined sections may beselected appropriately. A non-limiting example is the intervals of thescanning lines 182. The averaging of the apparent density may use adifferent averaging algorithm.

The controller 10 adds a density offset to the average density tocalculate the target density (step S4). The density offset is, forexample, set to a predetermined value. In this case, the target densityis calculated by adding the predetermined value to the average densitycorresponding to each position in the X-direction, as illustrated inFIG. 11. The predetermined value is set so that the target density isequal to or greater than the apparent density.

The controller 10 calculates the correction gain of each printingelement 181 (step S5). When the apparent density and the target densityhave a relationship such as the one in FIG. 11, then the correction gainof the printing element 181 b corresponding to point B, for example, isset to a value greater than 1. The correction gain of the printingelement 181 a corresponding to point A, for example, is set to 1. Therelationship between the correction gains of the printing elements 181 ato 181 e corresponding to points A to E is as follows: (point B, pointD)>(point C)>(point A, point E). When the correction gain of theprinting elements 181 a and 181 e is set to 1, the correction gain ofeach printing element 181 is set to at least 1.

After step S5, the controller 10 ends the procedure of the flowchart inFIG. 14. The controller 10 applies the correction gain of each printingelement 181 calculated with the procedure of the flowchart in FIG. 14 tothe multiplier 51 of the conversion block in FIG. 5. Application of thecorrection gain to the generation of the output pixel data allows theoutput pixel data to be corrected in accordance with the printingcharacteristics of the printing elements 181.

As described above, the printing apparatus 1 and printing correctionmethod according to the present embodiment set the correction gain ofeach printing element 181 to at least 1. This makes the printing resultobtained after application of the correction gain less prone tograininess.

Second Embodiment

A diffusion filter having the properties of an LPF can be used in colorreduction that is based on the error diffusion method in the firstembodiment. In the second embodiment, an LPF that takes into account thesize of the diffusion filter used in the error diffusion method can beapplied to the waveform of the pre-correction printing density.

The size of the diffusion filter can be indicated by the range overwhich the diffusion filter has an effect. The example diffusion filterin FIG. 8 has an effect from position 1 to position 5. The size of theexample diffusion filter in FIG. 8 can be considered to be five pixelsin the X-direction.

An LPF that takes into account the size of the diffusion filter can bethought of as being designed to have an effect over the same range as adiffusion filter. In the present embodiment, the LPF is assumed to takeinto account the size of the diffusion filter in FIG. 8. The LPF isdesigned to affect five pixels in the X-direction when applied to thewaveform of the pre-correction printing density.

As an example, the case of a high-frequency noise component beingincluded in the waveform of the pre-correction printing density,indicated by the dashed line in FIG. 15, is described. When an LPF thattakes into account the size of the diffusion filter is applied to awaveform that includes a high-frequency noise component, then thehigh-frequency noise component is removed from the waveform of thepre-correction printing density by application of the LPF.

For example, when a high-frequency noise component having a largeramplitude than the waveform of the pre-correction printing density isoverlaid on the waveform of the pre-correction printing density, thenthe high-frequency noise component affects the averaging of thepre-correction printing density. The density offset can become a largervalue than when no high-frequency noise component is present. Thedensity offset can be kept lower by removal of the high-frequency noisecomponent from the waveform of the pre-correction printing density.

The printing apparatus 1 according to the present embodiment can removethe high-frequency noise component by applying, to the waveform of thepre-correction printing density, an LPF that takes into account the sizeof the diffusion filter. The printing apparatus 1 according to thepresent embodiment can keep the density offset lower than when thedensity offset is set as in the first embodiment.

By applying an LPF that takes into account the size of the diffusionfilter, the printing apparatus 1 according to the present embodiment canlimit the scope of application of the LPF that leads to a reduction inthe sharpness of the output image data. Consequently, a reduction insharpness can be suppressed.

Third Embodiment

The correction gain applied in the multiplier 51 of the conversion blockin FIG. 5 can be treated as having a spatial distribution correspondingto each pixel. In this case, an LPF that takes into account the size ofthe diffusion filter can be applied not only to the waveform of thepre-correction printing density but also to the waveform indicating thespatial distribution of the correction gain. With this approach, thenoise component can be removed from the correction gain even after thecorrection gain has been calculated with the noise component remainingin the waveform of the pre-correction printing density.

As illustrated in FIG. 16, for example, the correction gain can berepresented as a waveform having a spatial distribution in theX-direction. The waveform indicated by the dashed line is an example ofa waveform that includes a noise component and has not yet beenprocessed by an LPF. The noise component can affect the correction gainthat is applied to the multiplier 51 of the conversion block in FIG. 5and the output pixel data generated by the conversion block. On theother hand, the waveform indicated by the solid line is an example of awaveform from which the noise component has been removed by applicationof the LPF. The removal of the noise component can suppress the effecton the output pixel data generated by the conversion block.

The printing apparatus 1 according to the present embodiment is assumedto treat the correction gain, applied to the generation of output pixeldata by the conversion block, as having a spatial distributioncorresponding to each pixel. An LPF that takes into account the size ofthe diffusion filter is assumed to be applied in the printing apparatus1 according to the present embodiment. This allows the printingapparatus 1 according to the present embodiment to remove the noisecomponent from the correction gain. Consequently, the density offset canbe kept even lower.

By applying an LPF that takes into account the size of the diffusionfilter, the printing apparatus 1 according to the present embodiment canlimit the scope of application of the LPF that leads to a reduction inthe sharpness of the output image data. Consequently, a reduction insharpness can be suppressed.

Even when the correction gain is applied to conversion based on theerror diffusion method, the correction gain is not applied to ahigh-frequency component at or above the cutoff frequency of thefiltering for error diffusion. Consequently, the application of an LPFto the correction gain tends not to affect conversion.

Fourth Embodiment

In the first embodiment, the correction gain can be set to resolve thedistribution of the printing density resulting from a shift in theprinting positions of dots printed by the printing elements 181. Severalmethods may be used to select the printing elements 181 for which thecorrection gain is set.

For the dot pattern illustrated in FIG. 10B, for example, the printingapparatus 1 may increase the weight of the correction gain set for theprinting element 181 c to resolve the low printing density portiondistributed in the scanning direction between point B and point C inFIG. 10B. Alternatively, the printing apparatus 1 may increase theweight of the correction gain of the printing element 181 b.

In the conversion block of FIG. 5, the filter 55 can be implemented byapplying a diffusion filter to the image data. In the second and thirdembodiments, an LPF that takes into account the size of the diffusionfilter can be applied to the waveform of the pre-correction printingdensity or the waveform indicating the spatial distribution of thecorrection gain. Like a diffusion filter, the LPF includes filtercoefficients that correspond to the horizontal positions of pixels inthe image data.

An example of the distribution of the filter coefficients of the LPF isillustrated in FIG. 17. In FIG. 17, the horizontal axis represents thehorizontal position of the pixel to be handled during conversion. Thevertical axis indicates the filter coefficient corresponding to eachposition. The distribution of the filter coefficients of the LPF is suchthat the filter coefficients have a local maximum at the centralposition in the horizontal direction, as illustrated by the solid rhombiand the solid line in FIG. 17, for example. The filter coefficients witha local maximum at the central position are also referred to as filtercoefficients with an unshifted central position. An LPF that has filtercoefficients with an unshifted central position tends not to affect thehorizontal distribution of the data for each pixel when the LPF isapplied to image data.

The distribution of filter coefficients of the LPF may be such that theposition at which the filter coefficients have a local maximum isshifted to the left, as illustrated by the empty rhombi and the dashedline in FIG. 17. The filter coefficients with a local maximum at aposition shifted from the central position are also referred to asfilter coefficients with a shifted central position. An LPF that hasfilter coefficients with a shifted central position may affect thehorizontal distribution of the data for each pixel when the LPF isapplied to image data. For example, when an LPF that has filtercoefficients with a shifted central position, as illustrated in FIG. 17,is applied to the waveform of the pre-correction printing density, theLPF shifts the entire waveform to the left. The waveform of the spatialdistribution of the correction gain is shifted to the left in accordancewith the shift to the left in the waveform of the pre-correctionprinting density. In other words, the application position of thecorrection gain is shifted to the left. The application position of thecorrection gain is also similarly shifted to the left when the LPF thathas filter coefficients with a shifted central position is applied tothe waveform indicating the spatial distribution of the correction gain.

In the example in FIG. 10A and FIG. 10B, an LPF that has filtercoefficients with the central position shifted to the left, asillustrated in FIG. 17, can be applied to the image data. The correctiongain for resolving the distribution of the printing density betweenpoint B and point C may be set to increase the weight for the printingelement 181 b or to increase the weight for the printing element 181 c.When the application position of the correction gain is shifted to theleft, the weights of correction gains corresponding to the pixels on theleft side among the printing elements 181 increase. In other words, theweight for the correction gain set for the printing element 181 b on theleft side is greater than the weight for the correction gain set for theprinting element 181 c on the right side. Conversely, when an LPF thathas filter coefficients with the central position shifted to right isapplied to the image data, the application position of the correctiongain is shifted to the right. In this case, the weights of correctiongains corresponding to the pixels on the right side among the printingelements 181 increase.

As described above, the printing apparatus 1 according to the presentembodiment can apply an LPF that has filter coefficients with a shiftedcentral position to image data. This allows selection of the printingelements 181 to which correction gain is to be applied during correctionof the output image data.

As in the diffusion filter in FIG. 8, for example, the position wherethe filter coefficients have a local maximum may be shifted to theright. Application of such a diffusion filter to image data yields abias in error diffusion towards pixels on the right, but this bias canbe compensated for by subsequently applying an LPF that has filtercoefficients with the central position shifted to the left. In otherwords, the printing apparatus 1 according to the present embodiment cancompensate for the property by which error diffusion increases to thepixels on the right during conversion by the error diffusion method orthe like. This approach allows image data converted by the errordiffusion method or the like to be corrected more appropriately.

The printing apparatus 1 and printing correction method according to thepresent disclosure can reduce an increase in graininess while correctingthe printing density.

Embodiments of the present disclosure have been described with referenceto drawings and examples. It is to be noted that various changes andmodifications will be apparent to those of ordinary skill in the art onthe basis of the present disclosure. Therefore, such changes andmodifications are to be understood as included within the scope of thepresent disclosure. For example, the functions or the like included inthe various components or steps may be reordered in any logicallyconsistent way. Furthermore, components or steps may be combined intoone or divided. While embodiments of the present disclosure have beendescribed focusing on apparatuses, the present disclosure may also beembodied as a method that includes steps performed by the components ofan apparatus. Embodiments of the present disclosure may also beimplemented as a method executed by a processor provided in anapparatus, as a program, or as a recording medium having a programrecorded thereon. Such embodiments are also to be understood as fallingwithin the scope of the present disclosure.

The invention claimed is:
 1. A printing apparatus comprising: a printercomprising a plurality of printing elements configured to print dots;and a controller configured to acquire input image data; acquire apre-correction printing density that is based on test pattern data andprinting characteristics of printing positions of the dots printed bythe plurality of printing elements, the test pattern data including auniform array of pixels; calculate a target density by averaging thepre-correction printing density; offset the target density so that thetarget density is equal to or greater than the pre-correction printingdensity; calculate a correction gain of the plurality of printingelements on the basis of a ratio of the target density to thepre-correction printing density; and control the printer on the basis ofthe correction gain and the input image data.
 2. The printing apparatusof claim 1, wherein the pre-correction printing density is a predictedprinting density that is predicted on the basis of the printingcharacteristics and the test pattern data.
 3. The printing apparatus ofclaim 2, wherein the controller is configured to control the printerwith a diffusion filter.
 4. The printing apparatus of claim 1, whereinthe pre-correction printing density is a measured printing densityacquired from a printing result of the test pattern data.
 5. Theprinting apparatus of claim 4, wherein the controller is configured tocontrol the printer with a diffusion filter.
 6. The printing apparatusof claim 1, wherein the controller is configured to control the printerwith a diffusion filter.
 7. The printing apparatus of claim 6, whereinthe controller is configured to apply the diffusion filter to thepre-correction printing density.
 8. The printing apparatus of claim 7,wherein a position where a filter coefficient is maximized in thediffusion filter is shifted from a center of the diffusion filter. 9.The printing apparatus of claim 6, wherein the controller is configuredto apply the diffusion filter to the correction gain.
 10. The printingapparatus of claim 9, wherein a position where a filter coefficient ismaximized in the diffusion filter is shifted from a center of thediffusion filter.
 11. An image correction method for a printingapparatus, the printing apparatus comprising: a printer comprising aplurality of printing elements configured to print dots; and acontroller configured to control the printer; the image correctionmethod comprising: acquiring, using the controller, a pre-correctionprinting density that is based on test pattern data and printingcharacteristics of printing positions of the dots printed by theplurality of printing elements, the test pattern data including auniform array of pixels; calculating, using the controller, a targetdensity by averaging the pre -correction printing density; offsetting,using the controller, the target density so that the target density isequal to or greater than the pre-correction printing density;calculating, using the controller, a correction gain of the plurality ofprinting elements on the basis of a ratio of the target density to thepre-correction printing density; and controlling, using the controller,the printer on the basis of the correction gain and the input imagedata.
 12. The image correction method of claim 11, wherein the pre-correction printing density is a predicted printing density that ispredicted on the basis of the printing characteristics and the testpattern data.
 13. The image correction method of claim 12, furthercomprising controlling, using the controller, the printer with adiffusion filter.
 14. The image correction method of claim 11, whereinthe pre -correction printing density is a measured printing densityacquired from a printing result of the test pattern data.
 15. The imagecorrection method of claim 14, further comprising controlling, using thecontroller, the printer with a diffusion filter.
 16. The imagecorrection method of claim 11, further comprising controlling, using thecontroller, the printer with a diffusion filter.
 17. The imagecorrection method of claim 16, further comprising applying, using thecontroller, the diffusion filter to the pre-correction printing density.18. The image correction method of claim 17, wherein a position where afilter coefficient is maximized in the diffusion filter is shifted froma center of the diffusion filter.
 19. The image correction method ofclaim 16, further comprising applying, using the controller, thediffusion filter to the correction gain.
 20. The image correction methodof claim 19, wherein a position where a filter coefficient is maximizedin the diffusion filter is shifted from a center of the diffusionfilter.